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I don't think it matters what module you use thats not how google page rank works. Im not 100% percent certain but I think one of the factors in deciding a pages pagerank is how important it is, i.e alot of forum topics probably have none or very little inbound links from other important pages so in Googles eyes the forum topics are not important and therefore are asigned a low pagerank or no pagerank.
For example my site main page has a PR of 4 but other pages that are deeper in the site, no matter what module they use, has a lower or unnasigned pagerank because not many other important pages link to it.
There is probably many other factors involved in how pagerank is assigned but I think what I mentioned above is one of them, i certainly dont think it has anything to do with what module you use.
Hope this helps
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